If the observed target is not perfectly symmetric, and exhibits actual
phase information (i.e. \(\Phi _O \ne 0\) , see for instance
[1]}), Eq. REF  is required.
Multiplying it with the left side operator \(\mathbf {K}\)  leads to a
new series of new phase-like quantities that are not contaminated by
instrumental phase, generalizing the notion of closure-phase
[2]} on which NRM-interferometry from the
ground [3]} and from space
[4]} rely entirely.
